[CONTRIBUTING] Add section on extension documents

Some changes to the ABI will require experimentation before the content
stabilises. Adding information to the main ABI is premature in this case.
However it is useful to document this information as an extension to the
ABI to enable further collaboration.

This section outlines the process for submitting an extension, what
criteria Arm use for accepting a contribution and how an extension can
move to the main ABI.

CONTRIBUTING.md

@@ -43,6 +43,41 @@ changes.
 If you want to make ABI changes that for some reason can't be discussed in
 public, you can send an email to [email protected].
 
+### Extension documents
+While the majority of new proposals can be added to existing
+documents. Proposals that extend the ABI, but are not yet stable are
+placed in an extension document. An example of an extension document
+is the PAuth Extension to ELF for Arm 64-bit Architecture. Extension
+documents have the following requirements:
+
+1. The document status must be Alpha.
+2. The document has an owner recorded in the table below. The owner
+   need not be from Arm.
+3. The document must not clash with other ABI extension documents, or
+   both extensions must be marked as being incompatible.
+
+
+The Arm approval process for accepting the extension is as follows:
+
+1. At least one person within Arm has reviewed and accepted the pull
+   request.
+2. There is a consensus within Arm that the extension can be added to
+   the ABI.
+
+Extension documents can move into the main ABI when the following conditions hold:
+
+1. The information in the document is stable.
+2. There is an implementation of the extension.
+3. The boundaries of when the extension applies are clear.
+
+An extension document that moves into the main ABI will add the
+necessary information to the main documents. In addition any design
+and rationale in the extension document will be moved to a new
+document in the design-documents folder.
+
+When the extension has either moved into the main ABI or has been
+withdrawn it will be moved to an archive folder.
+
 ## Manual checking of the PDF documents and Continuous Integration
 
 To check the outcome of your changes, run the `tools/rst2pdf/generate-pdfs.sh`